1. Field of the Invention
The present invention particularly relates to an input circuit of a satellite tuner for receiving satellite broadcasting.
2. Description of the Related Art
A parabolic antenna for receiving satellite broadcasting is attached with LNB (Low Noise Block Down Converter) for converting a signal from a satellite transmitted by, for example, 12 GHz band into a signal of a frequency of, for example, 1 GHZ through 2 GHz band. Power source for LNB is transmitted from a tuner side via a coaxial cable and an input stage of a tuner circuit of satellite broadcasting is provided with a circuit for supplying power source to LNB side.
FIG. 1 shows a constitution of an input stage of a conventional tuner 101 of satellite broadcasting. As shown in FIG. 1, the input stage of the conventional tuner 101 of satellite broadcasting is provided with an antenna terminal 102, a high pass filter 111, a choke coil 112 and a capacitor 113. The antenna terminal 102 is for connecting LNB 104 attached to a parabolic antenna 103 via a coaxial cable 105 and as the antenna terminal 102, for example, an F type connector is used. The capacitor 113 is a ceramic capacitor of a chip part of, for example, about 1000 pF.
A signal of, for example, 12 GHz band from a satellite is received by the parabolic antenna 103. The received signal is converted into a signal of, for example, 1 GHz through 2 GHz band by LNB 104 and is inputted to the antenna terminal 102 via the coaxial cable 105. Further, the input signal is supplied to an initial stage amplifier via the high pass filter 111.
Power source for LNB 104 is outputted through an LNB voltage supply line 114 and from the antenna terminal 102 via the choke coil 112 and is transmitted to LNB 104 via the coaxial cable 105.
The high pass filter 11 is provided for extracting a high frequency signal component inputted from the antenna terminal 102 and transmitting the high frequency signal component to the initial stage amplifier. The choke coil 112 is provided for cutting the high frequency signal component inputted from the antenna terminal 102. Further, the capacitor 113 is for removing a noise component included in the LNB voltage supply line 114. Particularly, in the case of a tuner of a digital satellite broadcasting, there is a case in which a clock component or the like plunges into the LNB voltage supply line 114. The capacitor 113 operates as a bypass capacitor for preventing a noise component of several hundreds MHz or lower such as a clock component from flowing to a signal path.
As described above, the input stage of the tuner 101 of satellite broadcasting is provided with the circuit for supplying power source to the side of LNB 104 and the choke coil 112 is provided between the antenna terminal 102 and the LNB voltage supply line 114 in series therewith such that a high frequency signal component does not flow to the side of the LNB voltage supply line 114. Further, the capacitor 113 of about 1000 pF is provided between the LNB voltage supply line 114 and the ground for the purpose of cutting noise.
The choke coil 112 is formed as a pattern on a microstrip in view of small size formation and no adjustment formation and accordingly, it is difficult to form a constitution having large inductance because of a space-related problem. Therefore, it is difficult to completely cut a high frequency signal component from the antenna terminal 102 by the choke coil 112 and there is a case in which a high frequency signal component inputted from the antenna input terminal 102 leaks to the LNB voltage supply line 114 via the choke coil 112.
Although the capacitor 113 is provided between the LNB voltage supply line 114 and the ground, according to the capacitor 113, there is used a constitution having comparatively large capacitance of about 1000 pF in order to remove noise of several hundreds MHz. However, in the case of such a capacitor having comparatively large capacitance, impedance with respect to a high frequency signal component is large and the high frequency signal component which leaks to the LNB voltage supply line 114 cannot be bypassed. Therefore, reflection occurs at a node NA, which deteriorates input VSWR (Voltage Standing Wave Ratio) characteristic.
That is, actually, the capacitor 113 is not only provided with a complete capacitance component but is provided also with an inductance component. Therefore, there is present a self-resonance frequency.
In the case of a capacitor of about 1000 pF, the inductance component becomes large and the self-resonance frequency becomes several hundreds MHz. Therefore, it seems that the capacitor 113 operates as an inductance component with respect to a high frequency signal of about 1 GHz. Therefore, the impedance of the node NA becomes large with respect to a high frequency signal. As a result, a high frequency component which flows in via the choke coil 112, leaks to the LNB voltage supply line 114 and reflection occurs at the point of the node NA which deteriorates the input VSWR characteristic.
The present invention provides a constitution capable of improving a receiving characteristic of an input unit.
According to the present invention, there is provided an input circuit of a satellite tuner comprising:
an antenna terminal for connecting to an outside converter via a cable;
a high pass filter for extracting a high frequency signal component from the outside converter transmitted via the cable and transmitting the extracted high frequency signal to an initial stage circuit;
a power source supply line supplied with a power source for providing a power source for the outside converter via the cable:
a choke coil inserted between the power source supply line and the antenna terminal for preventing the high frequency signal component from the outside converter transmitted via the cable and transmitting the power source from the power source supply line to the antenna terminal;
a first capacitor connected between the power source supply line and ground for removing influence of a noise component included in the power source transmitted to the power source supply line; and
a second capacitor connected between the power source supply line and the ground for removing influence of the high frequency signal component which leaks in via the choke coil.
There are provided the capacitor of about 1000 pF for removing noise and the capacitor of about 1 pF for flowing the high frequency component which leaks to the LNB supply line to the ground between the LNB voltage supply line and the ground. By operation of the capacitor of about 1 pF, impedance of a node NA between the LNB voltage supply and the choke coil is reduced in a broad band and deterioration of input VSWR can be prevented.